Adding manually an FTP user for a specific folder

Is it possible to add a specific user allowed to read/write by FTP in a specific folder inside ISPConfig ? I know it cannot be done with the current version and the control panel, but can I modify directly my /etc/passwd to add the user, then let him access by proftpd ?

I host several small website (without domains for them, only subfolders on my domains), I just want to permit access to them to the end user...

1. adduser webx_specificuser
2. gave real name and password
3. edited /etc/passwd to change the :/home/webx_specificuser to reflect my correct path (ie : /var/www/webx/web/myspecificfolder/)
4. edited /etc/shadow, but i guess no changes were made here.
5. added webx_specificuser to the end of /home/admispconfig/ispconfig/users
6. chown -R webx_specificuser:www-data /var/www/webx/web/myspecificfolder
7. Bingo, I have correct access to a folder inside a domain for a user.

The only thing I wonder is why put the new user at the end of /home/admispconfig/ispconfig/users ?